Optical metrology systems, such as reflectometers, often use refractive optics to focus the radiation onto the sample being measured. With the sample positioned in the focal plane of the optical metrology system, the metrology system can measure the desired parameters of the sample, including, but not limited to such things as the thickness and optical characteristics of the film or films on the sample, as well as critical dimensions (CD). Accordingly, prior to measuring a sample, optical metrology systems generally attempt to place the sample at the focal plane of the optical system. Imprecision in the focusing process contributes to a decrease in the precision and repeatability of the measurement results. In the case of measurement systems that use multiple wavelengths of light, refractive focusing optics will intrinsically produce a variation in actual focal length as a function of wavelength. Conventional measurements at a single (e.g. average) focal plane in a multiple wavelength system causes a significant number of wavelengths to be slightly out of focus, resulting in increased spot size and decreased intensity. Thus, improvements are desired.